Earth’s Magnetic Shield is Older than Previously Thought

Plate Tectonics Got Started Early in our Planet’s History

Our planet has a magnetic field, similar to the magnetic field that can be generated by a simple bar magnet. This field is aligned close to but not directly with the Earth’s axis of rotation. The magnetic poles lie some way from the geographic poles (separated by approximately eleven degrees). Earth’s magnetism is powered by fluid motion inside our planet and this field protects all life from the harmful solar winds that are expelled by the sun. When this field was first generated has remained a mystery. It had been thought that the Earth’s magnetic field had been around for some 3.45 billion years, now new research conducted by scientists at the University of Rochester (New York) suggests that this magnetic field is actually much older.

John Tarduno, a geophysicist at the University of Rochester and lead author of a paper published in the journal “Science” estimates that the Earth’s magnetic field was formed at least four billion years ago. This is much earlier in our planet’s history than previously thought, it is estimated that Earth was formed some 4.56 billion years ago.

Since 2010, the best estimate of the age of Earth’s magnetic field has been 3.45 billion years. But now a researcher responsible for that finding has new data showing the magnetic field is far older.

Why is a Magnetic Field Important?

It has to do with our sun and cosmic radiation. As well as visible light, the sun sends out streams of charged particles into space, it is not the only source of cosmic radiation, any luminous body in the universe produces radiation, but since the sun is 93 million miles away, at the centre of our solar system and with a radius of approximately 700,000 kilometres it is the biggest contributor to the harmful cosmic rays that get sent our way. Particles of different wavelengths and energies are being generated all the time by our sun and it is these higher energy radioactive particles that are dangerous to life forms. Thankfully, those that make it to Earth are deflected away by our planet’s strong magnetic field. This magnetism acts as a shield helping to protect our planet and the life that exists on it.

An Artist’s Illustration of the Earth’s Magnetic Field Deflecting Particles

Helping to keep our planet habitable.

Picture Credit: Michael Osadciw/University of Rochester

Professor Tarduno explained:

“A strong magnetic field provides a shield for the atmosphere. This is important for the preservation of habitable conditions on Earth.”

This life saving magnetic field is also responsible for producing the Aurorae (northern lights – Aurora Borealis and southern lights Aurora Australis), spectacular light shows that are created when the solar wind and its charged particles interacts with our planet’s magnetic field and atmosphere. As these particles approach Earth, they distort our magnetic field and allow some charged, high energy particles to enter our atmosphere at the magnetic north and south poles. These charged particles interact with gases in our atmosphere and “excite” the gas particles which in turn glow, just like the gas in a florescent tube light you might have in your kitchen.

A Spectacular Light Show (Northern Lights Seen in North Yorkshire)

Aurora Borealis seen in northern England.

Picture Credit: Owen Humphreys/PA

Our planets magnetism is generated in its liquid iron core. It acts as a “geodynamo” and requires regular releases of heat from our planet to function. This heat release is aided by plate movements at the Earth’s crust. Convection allows the transfer of heat from the interior to our planet’s surface, but the origin of our tectonic plates is contentious, with many physicists suggesting that our planet lacked a magnetic field for more than a billion years after it was formed.

A study of the mineral magnetite found within zircon crystals collected from the ancient rocks of the Jack Hills of Western Australia has helped the Rochester University to determine that the Earth’s magnetic field is older than previously thought at around 4 billion years of age.

Magnetite is a naturally occurring magnetic iron oxide, it locks in information about the Earth’s magnetic field as it cools and forms from its molten state. The ancient rock deposits of the Jack Hills represent some of the oldest strata on our planet and zircons from these rocks have already been used to help determine how quickly the Earth cooled after its initial formation.

In order for the team to get reliable, accurate results, it was crucial that the minerals remained unchanged over the vast period of time since their formation. Professor Tarduno’s study of the magnetic field strength preserved inside the pristine zircon crystals has enabled the team to build up a picture of the Earth’s magnetic field over time. The microscopic zircons were analysed using a superconducting, quantum interference device, which is unique to the University, the sensitive instrument (called a SQUID magnetometer), showed that the intensity measurements recorded in the samples were indeed as old as four billion years.

Implications for Life on Earth and Other Planets

The intensity measurements reveal information about the presence of a “geodynamo” at the Earth’s core. Tarduno explained that solar winds could interact with the Earth’s atmosphere to create a small magnetic field, even in the absence of this core dynamo. Under those circumstances, it has been calculated that the maximum strength of a magnetic field would be 0.6 μT (micro-Teslas). The values measured by Professor Tarduno and his colleagues were much higher than 0.6 μT, suggesting the presence of a “geodynamo” at the planet’s core, as well as indicating the existence of the active plate tectonics needed to release the built-up heat.

Professor Tarduno added:

“There has been no consensus among scientists on when plate tectonics began. Our measurements, however, support some previous geochemical measurements on ancient zircons that suggest an age of 4.4 billon years.”

Four billion years ago, our sun was much younger and it was sending out much more powerful solar winds which were up to a hundred times stronger than today’s. In the absence of this nascent magnetic field the high energy particles that make up the solar wind would have ionised and blasted away light elements (those with a relatively low atomic mass), from the atmosphere, elements such as hydrogen, nitrogen, carbon and oxygen. The loss of these elements would have made the evolution of complex life on our planet almost impossible (probably completely impossible).

Our planet, may have come to resemble that of our second nearest planetary neighbour, Mars. Mars is approximately half the size of Earth, the mass of Mars is around ten percent of the Earth, it may once have had an active “geodynamo” for a time after its formation, but due to the planet’s size, this active, convectional dynamo seems to have run out of energy and ceased. This led to the Red Planet losing any magnetic field that it had. The atmosphere was blasted away by the solar wind. Today, scientists are confident that Mars once had liquid water but it is likely that this water was driven off along with most of the light elements in the atmosphere and on the surface of the planet.

Professor Tarduno believes that the loss of the “geodynamo” dramatically altered the history of Mars. He stated:

“It may also be a major reason why Mars was unable to sustain life.”

Mars does have an atmosphere, it consists of nitrogen and carbon dioxide but it is just one percent the thickness of our own planet’s atmosphere. Unmanned, robotic vehicles like the Mars Rover may have provided us with evidence that Mars may once have had vast amounts of liquid water – so much water in fact that there is evidence of global scale floods on its barren, rocky surface.

It seems we have a lot to be grateful for when it comes to our magnetic field and its early establishment may have assisted in the development of life on our own planet.